Liquid-crystalline materials having cholesteric properties, herein abbreviated as "cholesteric liquid crystals", are substances having a helical arrangement of the molecules. These materials are usually prepared as a thin layer between two suitable substrates in such a way that the helix axis is perpendicular to the substrate surfaces. The pitch of the helix is material-dependent and is constant over the layer thickness. Such optically anisotropic layers are able to reflect a circular light component fully if the direction of rotation and light wavelength .lambda. in the material correspond to the direction of rotation and pitch p of the cholesteric helix (cholesteric reflection). By contrast, the second circular light component having the opposite direction of rotation is transmitted fully.
The cholesteric reflection occurs in a spectral band between the wavelengths .lambda..sub.l =p*n.sub.o and .lambda..sub.2 =p*n.sub.e, where n.sub.e and n.sub.o denote the extraordinary and ordinary refractive indices of the material. This reflection band can be characterized by two parameters, the central wavelength .lambda..sub.o and the width .DELTA..lambda.. The central wavelength .lambda..sub.o depends on the mean refractive index and pitch p of the material. The width .DELTA..lambda. of the cholesteric reflection band is dependent on the birefringence .DELTA.n=n.sub.e -n.sub.o of the material in accordance with the equation .DELTA..lambda.=p*(n.sub.e -n.sub.o). In practice, the birefringence of most cholesteric materials in the visible spectral region is restricted to values lower than 0.3. Consequently, the maximum possible band width is about 100 nm. Usually, however, only 30-50 nm is achieved. Outside the reflection band and in the absence of absorption, light having both polarization directions (right-handed circular and left-handed circular, i.e. unpolarized) is transmitted fully. The reflected or transmitted circular-polarized light can, if desired, be converted into linear-polarized light by means of an additional quarter-wave retardation layer.
An essential prerequisite for the use of cholesteric materials is adequate thermal and mechanical stability of the layers. This stability can be achieved by fixing the alignment state by polymerization or by rapid cooling to temperatures below the glass transition temperature. Stable cholesteric layers of this type are described, for example, by R. Maurer et al. under the title "Polarizing Color Filters made from Cholesteric LC Silicones" in SID 90 DIGEST, 1990, pp. 110-113.
Owing to the aforementioned optical and mechanical properties, cholesteric materials are suitable both as polarizing and color-selective reflectors and as polarizing and color-selective optical filters. They have the great advantage over filters made from absorbent materials in that heating of the filter material is substantially avoided. Given a corresponding band width of the cholesteric reflection, these materials can also be used as so-called reflective polarizers, for example in liquid-crystal displays:
If unpolarized light from a light source located between a cholesteric layer and a mirror (metal) hits the cholesteric layer, circular-polarized light having a direction of rotation opposite to that of the layer helix passes through the layer, while the remaining fraction having the same direction of rotation is reflected. This component hits the mirror and experiences inversion of the direction of rotation of the circular polarization, with the consequence that this light component can then likewise pass through the cholesteric layer. In theory, therefore, complete conversion of unpolarized light into circular-polarized light takes place. Compared with conventional arrangements consisting of light source, mirror and absorptive polarizer, it is possible to double the light yield of the illumination unit of a liquid-crystal display. At the same time, the absence of absorption means that heating and bleaching of the polarizer is avoided (S. V. Belayev, M. Schadt, M. I. Barnik, J. Funfschilling, N. V. Malimoneko and K. Schmitt, JPN. J. APPL. PHYS. 29, L273 (1990)).
Photopolymerizable cholesteric materials can also be photostructured. This is described, for example, by R. Maurer et al. "Cholesteric Reflectors with a Color Pattern" in SID 94 DIGEST, 1994, pp. 399-402. The material described therein exhibits pronounced thermochromicity, i.e. a strong dependence of the reflection color on temperature. The desired color can therefore be set by means of the temperature of the sample and fixed by exposure to UV through a mask. The color of the unexposed areas of the cholesteric layer can be modified by subsequent temperature change. This color is permanently fixed by a second exposure to UV, if desired again through a mask. This operation can be repeated at different temperatures with further masks to produce multicolored structured filters and reflectors. Such structured filters and reflectors can be used, for example, in color projectors and in liquid-crystal displays.
A further application of cholesteric materials is as pigments produced by grinding and screening cholesteric films. Suitable materials and their production are described, for example, in EP 0 601 483.
The actual achievement of these potential applications has hitherto been greatly restricted by the limited width of the reflection bands. For industrial use, it is in addition desirable for both the central wavelengths of the reflection band and the width of the reflection band to be freely and independently adjustable in accordance with the particular requirements. For the specific use as reflective broad-band polarizers, it is even necessary for the reflection band to cover the entire visible spectral region, i.e. for the cholesteric layer to have a band width of greater than 300 nm.
The problem of inadequate band width can in principle be solved by constructing the optical element from a plurality of layers having different central wavelengths. This is described in the above-mentioned article by R. Maurer et al. However, this method is very expensive and has the disadvantage that the optical quality of the optical element decreases with each additional layer owing to scattering at flaws and inhomogeneities.
Another process of solving the above-mentioned problem is to broaden the reflection band by means of a gradient in the helix pitch (pitch gradient). This approach has already been known for some time from theoretical studies (for example, S. Mazkedian, S. Melone, F. Rustichelli, J. PHYSIQUE 37, 731 (1976) and L. E. Hajdo, A. C. Erigen, J. OPT. SOC. AM. 36, 1017 (1979)).
The process described in EP 0 606 940 A2 uses a mixture of chiral and nematic monomers having different reactivity with respect to their polymerization properties, the mixture additionally containing a dye whose absorption properties are matched to the UV radiation used for the photopolymerization. During the photopolymerization, the dye absorbs part of the UV light, generating a strong intensity gradient within the cholesteric layer. Owing to the different reactivity of the nematic and chiral monomers, a diffusion process takes place, generating the desired pitch gradient. In EP 0 606 940 A2, this is a linear pitch gradient, where the smallest pitch occurs on the side facing the UV source. The process described is furthermore characterized by continuous UV exposure to low intensities for a long period.
A disadvantage of this process is that it always requires a mixture of various monomers having different reactivity with respect to polymerization and in addition a dye must be incorporated. This process thus requires complex and expensive material synthesis. A further disadvantage is that the ultra-violet exposure must be kept constant for a relatively long time, in the order of 10 minutes. In the continuous production process, in which the optical layer is applied continuously to or between films and photopolymerized, a long, homogeneously illuminated exposure zone is therefore necessary. The long residence time greatly restricts the achievable throughput of produced film. The admixture of the UV dye also results in some disadvantages. For example, the absorption of the dye, as described in one example of EP 0 606 940 A2, results in a undesired restriction of the band width in the short-wave spectral region. In addition, the warming associated with dye absorption can result in impairment or even destruction of the optically active layer.
A further process which likewise has the object of generating a pitch gradient has been published by Faris et al., "A Single-Layer Super Broadband Reflective Polarizer" in SID 96 DIGEST, 1996, pp. 111-113. This process is based on a mixture of a photocrosslinkable cholesteric polysiloxane with a non-crosslinkable low-molecular-weight nematic compound. Here too, slow photocrosslinking is carried out with low-intensity UV exposure, with phase separation between the crosslinkable polysiloxane and the non-crosslinkable nematic compound taking place during the UV polymerization. As a consequence of this phase separation, the segregated molecules can diffuse within the layer and generate a concentration gradient, which in turn results in a pitch gradient.
As in the previous process, this process also has the principal disadvantage that at least two different starting components must be synthesized. This process is likewise based on slow crosslinking being achieved by maintaining the UV exposure for an extended period, with the disadvantages already described above for a continuous production process.